(i) Technical Field of the Invention
The present invention relates to an apparatus and a method for the mirror surface grinding of a magnetic disc substrate for a hard disk.
(ii) Description of the Related Art
A hard disk for a computer is a magnetic disk prepared by applying a magnetic substance to the surface (one side or both sides) of a disk-like substrate (e.g., made of aluminum), and reads information by rotating it at a high speed (e.g., 10000 rpm or faster) and moving a head along the magnetic surface.
The degree of parallelism (precision of the thickness) and the surface coarseness of substrate for a hard disk (hereafter, magnetic disk substrate) requires high precision to record information in a high density. For example, small magnetic disk substrates of a diameter of 2.5 inches (ca. 64 mm) and a diameter of 3.5 inches (ca. 95 mm) have a maximum acceptable range (difference between maximum and minimum thickness), for example, of 3 .mu.m and 7 .mu.m, respectively.
Conventionally for processing magnetic disk substrates of such high precision, a flatbed lap has been mainly used. The flatbed lap is a lap for simultaneous lapping both sides and has, for example, gear teeth surrounding a part for fitting a workpiece; the fitting part rotates by itself meshing with the central and surrounding gears, pressure is applied by an upper cylinder, and a free abrasive is put between a lap tool (or lapping machine) and the workpiece is processed by relative movement of the lap and the workpiece.
The aforementioned conventional lapping machine (e.g., a flatbed lap) is characterized by possible high processing precision by using relatively simple facilities. On the contrary, there are following problems.
(1) The conventional lap, when applied to processing using a free abrasive, results in a very low processing rate (1/10 or lower grinding rate) and is time consuming for processing. Therefore, a facility of a large size is normally used for simultaneous processing of a plurality of magnetic disk substrates. Notwithstanding this, the processing time required for a disk is longer.
(2) A standard lap is previously processed in high precision to process a workpiece to fit to the lap. Therefore, maintaining the precision of the lap itself, which is lowered by abrasion, etc. requires repeated processing of the lap. Normally in this case, "lapping adjustment" of the upper and lower laps is carried out for lapping the respective laps. By this step, uneven surfaces become even. However, the right angle against the rotational shaft cannot be kept constant, which lowers the degree of parallelism of the workpiece after processing. This means that constant pressure lapping allows truing (lapping adjustment) on a lapping machine; however, it does not allow readjustment of the degree of parallelism of a jig for holding the workpiece to a grinding wheel even if the flatness of the grinding wheel (the lap) has been improved. Therefore, the degree of parallelism (precision of thickness) of both sides of the workpiece cannot be improved by the adjustment.
In addition, very recently, crashing of hard disks, particularly of motors has very frequently occurred. One of the causes of disk crashing is an unbalanced thickness of a magnetic disc substrate. There is a problem that the unbalanced thickness of a magnetic disc substrate causes a decentering force which lowers the life of the bearings of the motor, finally resulting in crash of the motor in a short time. Thus, in order to increase the reliability of the hard disk, increasing the degree of parallelism (precision of thickness) of the magnetic disc substrate is required more than with conventional substrates. However, the conventional lap aforementioned does not achieve this object due to the requirement of a longer processing time that is not practical.